In order to achieve very high fuel utilization in a proton exchange membrane fuel cell power plant, such as on the order of 99%, while avoiding fuel starvation in any part of the fuel cell power plant, a fuel recycle loop is commonly employed. Such a recycle loop receives the fuel exhaust from the fuel cell power plant, and returns it to the fuel inlet of the fuel cell power plant, utilizing a pump of some sort to restore the pressure of the recycled fuel to a pressure substantially the same as the pressure of the fresh, inlet fuel. The use of a fuel recycle loop allows the fuel utilization in the overall system to be higher than the fuel utilization in the individual fuel cells.
Recycling fuel requires a blower or compressor which adds costs and consumes parasitic power. In the case of pure hydrogen systems, in contrast with reformate, the fuel is difficult to pump due to the low density of hydrogen.
Alternatively, high fuel utilizations may be achieved by passing the fuel serially, first through one set of fuel flow fields, and then through a next set of fuel flow fields in turn, either within each individual fuel cell or through different groups of fuel cells in a fuel cell stack, which is referred to as a “cascaded” fuel cell system. Such systems can achieve overall system fuel utilization on the order of 99% without using a fuel recycle loop, but such systems present other problems. These problems include a high pressure drop across the fuel flow fields; a risk of local fuel starvation, especially in the last of the fuel flow fields, since the hydrogen is diluted by inert gases such as nitrogen that migrates through the electrolyte; and difficulty in startup or shut down of the fuel cell system because of the problem of introducing fuel rapidly throughout the system, particularly on startup.
The startup problem has been partially solved by introducing the fuel into all of the fuel flow fields directly from the fuel supply pipe and extracting the fuel from all of the fuel flow fields directly into the exhaust, as disclosed in U.S. Pat. No. 6,821,668.